A parasitic cuckoo chick foisted upon other birds can turn out to be luck in disguise, saving the nest with a disgusting defense.

About 1 percent of bird species, including cuckoos, outsource their childcare by sneaking into other birds’ nests and leaving an egg.

The intruder chick often kills or outcompetes the rightful offspring of a nest. Defense by cuckoo chicks of carrion crow nests at high risk of predator attack could be the first example of a parasitic bird’s benefit to its host, says ecologist Daniela Canestrari of the University of Oviedo in Mieres, Spain.

Chicks of the great spotted cuckoo (Clamator glandarius) don’t directly kill chicks of the carrion crows (Corvus corone corone), so the crow parents have a chance of producing some of their own offspring. When lots of predators lurk about, the cuckoo chick’s strong defense mechanism — emptying its bowels of a caustic, stinking slime — may explain why parasitized nests are more likely to fledge chicks than unparasitized ones. In the right circumstances, even cuckoo parasitism can turn into a mutually beneficial relationship, Canestrari and her colleagues argue in the March 21 Science.

To have an example of a benefit of brood parasitism is “exceedingly unusual and cool,” says evolutionary biologist Claire Spottiswoode at the University of Cambridge, who has worked with brood parasites.

A previous suggestion of a brood parasite’s benefit, reported nearly 50 years ago from giant cowbirds parasitizing oropendolas nests in Panama, hasn’t stood up to later research, says another researcher who studies cuckoos, Juan Soler of the Arid Zones Experimental Station in Almería, Spain. Supposedly the cowbird chicks would pick parasitic botflies off the rightful nestlings. Later researchers dismissed the idea, questioning whether the young cowbirds could even grasp and yank off the parasites.

In the new study, researchers analyzed the fates of 741 carrion crow nests at a rural study site in Spain over the course of 16 years. Nests with at least one cuckoo chick in them were more likely (76 percent versus 54 percent) to succeed in fledging at least one of the crow nestlings than nests with no cuckoo chicks were. There was a cost, though: Among the nests that survived, those with cuckoo chicks fledged on average only 2.1 crows, fewer than the 2.6 fledglings in nests without an intruder..

To see if cuckoos were just targeting the most robust crow nests, researchers added and removed cuckoo chicks. The nests with cuckoos were still more likely to fledge at least one crow.

When disturbed by intrusions such as a researcher grabbing a nestling, spotted cuckoo chicks at least 3 or 4 days old defend themselves — and everything else within splattering distance — by releasing dark, sticky glop that’s much stinkier than their usual poop. “It’s terrible,” Canestrari says. “The worst part is that the smell doesn’t go away when you wash but lingers for a long time.”

Chemical analysis of the components revealed what coauthors called “an acid bomb,” she says. It contains substances known to repel animals, and the researchers’ own tests with predators such as falcons confirmed the slime’s power to disgust. Only one of 12 feral cats tried even a bite of meat coated in cuckoo excretions. “It must have been very hungry,” Canestrari says.

“I don’t think that the results are broadly applicable to brood parasites in general,” says Stephen Rothstein of the University of California, Santa Barbara. Nearly all other species of parasitic cuckoos and parasitic honeyguides kill all of the host young, he says. Even with parasitic cowbirds, “which are sometimes mistakenly cited as examples of parasites that are not very harmful,” he says many host species nearly always lose all of their own young because cowbirds hatch sooner or are larger, and thus outcompete the host chicks for food.